The following Patent Literature 1 discloses this kind of power transmission apparatus. The power transmission apparatus disclosed in Patent Literature 1 includes a first rotor having a magnet and mechanically connected to driving wheels, a second rotor having a winding electromagnetically coupled with the magnet of the first rotor and mechanically connected with an engine (prime mover), a stator having a winding electromagnetically coupled with the magnet of the first rotor, a transformation-rotor having a winding electrically connected to the winding of the second rotor and mechanically connected to the second rotor, and a transformation-stator having a winding electromagnetically coupled with the winding of the transformation-rotor. In Patent Literature 1, the power transmitted from the engine to the second rotor is transmitted to the first rotor through the electromagnetic coupling between the winding of the second rotor and the magnet of the first rotor. Therefore, the driving wheels can be driven by the power of the engine. Furthermore, electric power supplied from a battery via an inverter to the winding of the transformation-stator can be supplied to the winding of the transformation-rotor and the winding of the second rotor through the electromagnetic coupling between the winding of the transformation-stator and the winding of the transformation-rotor. Therefore, the rotational speed of the driving wheels can be controlled by controlling electric power to be supplied to the winding of the transformation-stator. Moreover, through the electromagnetic coupling between the winding of the stator and the magnet of the first rotor, the first rotor can generate power using the electric power supplied from the battery to the winding of the stator via the inverter and can drive the driving wheels. Therefore, the torque to be transmitted to the driving wheels can be controlled by controlling electric power to be supplied to the winding of the stator.
In Patent Literature 1, a winding axis of the winding of the transformation-rotor and a winding axis of the winding of the transformation-stator are parallel to the rotational axes of the rotors, and are perpendicular to a radial direction along which the transformation-rotor and the transformation-stator are opposed to each other. Therefore, the transformation-rotor and the transformation-stator can simply function as a transformer. No torque is generated between the transformation-rotor and the transformation-stator. Accordingly, the transformation-rotor and the transformation-stator cannot receive the torque generated by the engine. The torque generated by the engine is received as only the torque being generated between the first rotor and the second rotor. As a result, torque transmission capacity decreases.
In the Patent Literature 1, when the power of the engine is used to drive the driving wheels, if the rotational speed of the first rotor connected to the driving wheels exceeds the rotational speed of the second rotor connected to the engine, induced current flows through the winding of the second rotor according to a rotational speed difference between the first rotor and the second rotor, and a torque (braking torque) acts on the first rotor (driving wheels) in such a way as to reduce its rotational speed. In this case, if electric power is supplied to the winding of the second rotor by supplying electric power from the battery to the winding of the transformation-stator via the inverter, the braking torque can be prevented from acting on the first rotor. However, an inverter is required to be provided between the battery and the winding of the transformation-stator to perform a bidirectional electric power conversion, i.e., an electric power conversion for converting the DC power of the battery into AC power to be supplied to the winding of the transformation-stator (winding of the second rotor) and an electric power conversion for converting the AC power of the winding of the transformation-stator into DC power to be recovered by the battery. As a result, not only the inverter that performs the bidirectional electric power conversion between the battery and the stator winding but also the inverter that performs the bidirectional electric power conversion between the battery and the transformation-stator winding are separately required. If two inverters are provided, the configuration of the apparatus becomes complicated and the cost for the apparatus increases.    Patent Literature 1: JP 3067594 B    Patent Literature 2: JP 2007-116837 A    Patent Literature 3: JP 9-46815 A